The present invention relates to the field of drilling from a water surface or level and more specifically to mining or oil industry-type drilling operations for the purpose of carrying out rapid explorations of sites with a view to the possible subsequent working thereof.
An essential condition for carrying out such processes consists of exerting on the drilling tool a bearing stress, which is constant and normal to the surface to be perforated. This condition is obviously difficult to fulfil, when drilling takes place from a water surface and in this case only two solutions have been known up to now.
According to the first solution, a floating drilling platform is brought onto the site and then after installing pillars resting on the sea bed, the platform is raised above the water surface and it rests on the pillars. Thus, the same situation arises as when drilling on land.
According to the second solution, a barge is brought to the site and is anchored there. The top of the drill pipe string is equipped with a complicated swell or surge compensating apparatus, which serves to give the drill pipe string a vertical displacement equal and opposite to that of the barge, so as to obtain an overall zero displacement variation on the tool. In other words, the bottom point of the drill pipe string is fixed relative to the ground and the top point, which is linked with a moving point, is fixed in space.
These two types of drilling systems, which are essentially used in oil drilling operations, are costly at the initial investment stage, from the maintenance standpoint and in use.
Moreover, the drilling systems of the second type with pounding compensation, pounding being the phenomenon due to the cyclic action of variable vertical pressure stresses on the bottom point of the drill pipe string, are only effective when the string is relatively elastic, i.e. when it has a considerable length, so that in the case of limited water depths, it is more difficult to obtain compensation and drilling is less efficient.